The present invention relates to rectifiers, and in particular to a method and an apparatus for optimizing the output power of a rectifier comprising a preregulator and a DC/DC converter.
In such rectifiers, the output power of the preregulator is the input power to the DC/DC converter. The input power to the preregulator is the power drawn from the mains. This input power P=Uxc2x7I, I being the input current and U being the input voltage, is dependent of the actual load on the output. Typically, the input voltage U is dependent in the environment and the input current I is varied to compensate for this and to achieve the desired power. If the input voltage drops, the input current I is increased, but must, for electrical and thermal reasons, be kept below a maximum limit. This, a minimum voltage is required to ensure a certain power level.
To guarantee full operation at all specified conditions a nominal maximum output power is defined. This power is used as a limit regardless of the actual input voltage. For input voltages higher than the minimum, therefore, the preregulator will not deliver its theoretical maximum output power.
Even if the output power from the preregulator at a given time would be sufficient to sustain a higher output power from the DC/DC converter than the nominal maximum output power level, the output power is kept at the nominal maximum power level. The input power is matched with the output power to make sure that it is sufficient at the specified input conditions. The nominal maximum output is always the same even if the input conditions are better. Thus, the input stage is not used at its real maximum potential when the conditions are good, that is, when the input voltage is high.
It follows that the output power that could be obtained from the rectifier is in many cases higher than the nominal maximum output power. When the possible output power from the preregulator is below the required output power, the rectifier delivers no output power at all, even though it would be able to deliver a power level lower than the predefined power level. When the conditions are more favourable, the rectifier still only delivers the predefined power level even though a higher power level could be delivered.
Prior art rectifiers are known that function when the available output power from the preregulator is below the nominal output power. If, however, the output power from the DC/DC converter is too high to be maintained by the output power from the preregulator, these prior art rectifiers will cease to function in an uncontrolled way.
It is an object of the present invention to enable an output power from a rectifier comprising a preregulator and a DC/DC converter, that is the maximum possible output power for the current conditions at any given time.
This object is achieved according to the present invention by a rectifier of the specified type, comprising means for controlling the output power POUT from the DC/DC converter in dependence of the input power to the preregulator.
A method of controlling such a rectifier by controlling the output power POUT in dependence of the input current IIN is also disclosed.
This solution ensures that the output power from the rectifiers is never higher than what the preregulator can supply. As a result the rectifier according to the invention may be used at any power level and any input conditions.
The output power POUT may be controlled by controlling the output current IOUT from the DC/DC converter.
A status signal indicating if the input current IIN is at its maximum may be obtained in several different ways:
Based on the input current IIN to the preregulator. In this case, the rectifier comprises
means for measuring the input current IIN to the preregulator;
means for comparing the input current IIN to a maximum allowed input current to obtain the status signal.
Based on an input current reference value. In this case the rectifier comprises
means for obtaining a reference value IINREF for the input current IIN for the current conditions;
means for comparing said reference value to a reference value IREF to obtain said status signal.
Based on a control voltage for the output voltage UINT of the preregulator. In this case the rectifier comprises:
means for measuring the output voltage UINT from the preregulator;
means for generating a control voltage UCTRL by comparing the output voltage UINT to a reference voltage UINTREF;
comparing means for comparing said reference voltage UCTRL to a reference voltage UREF1 to obtain said status signal.
Based on the output voltage UINT of the preregulator. In this case the rectifier comprises
means for measuring the output voltage UINT from the preregulator;
means for comparing said output voltage UINT to reference value UREF2 to obtain said status signal.
Preferably, the status signal is processed before it is fed as a control signal to the DC/DC converter.
The processing may include
filtering in a filter unit
obtaining a difference signal between the filtered signal and an output current reference signal (IOUTREF);
comparing the difference signal to the output current IOUT from the DC/DC converter, the result of the comparison constituting the control signal to the DC/DC converter.
With the inventive solution the two feedback loops used in prior art solution are replaced by one. This enables a maximum output power for all operating conditions.